In previous video coding standards like MPEG-1, MPEG-2, MPEG-4 or MPEG-4 AVC, a compressed picture is usually divided into a number of rectangle units known as macroblocks or coding units. A macroblock is usually defined as a two dimensional block of image samples of 16 pixels by 16 pixels for luminance (or luma) samples. For each macroblock, the compression ratio for the macroblock is controlled by a quantization parameter (or quantization scale parameter) and the quantization parameter is usually coded in a header of the macroblock. The value of the quantization parameter coded in the header of the macroblock is usually expressed as a differential value with respect to the quantization parameter of the previous macroblock in coding order. The quantization parameter expressed in such a differential value may be referred to as a delta quantization parameter.
Similarly in the previous video coding standards, the prediction coding type parameter, which indicates if the prediction samples for a block of image samples (i.e., a macroblock) are spatially predicted (intra predicted) or inter picture predicted (inter predicted), is also signalled in the units of a macroblock.
FIG. 1 depicts a diagram illustrating a largest coding unit (LCU) 102 comprising coding units (CUs) 104 coded based on the latest video coding standards that support variable size coding units, for example, High Efficiency Video Coding (HEVC). As shown, a coding unit 104 can be coded as an intra predicted block (depicted as shaded blocks) 106 or an inter prediction block (depicted as unshaded blocks) 108. In the example shown in FIG. 1, the prediction mode type parameter 110 for signalling or indicating whether a coding unit 104 is an intra predicted or inter predicted block is coded at a smaller coding unit than the coding unit 102 carrying the delta quantization parameter 112. That is, as depicted in FIG. 1, the delta quantization parameter 112 is coded at the LCU level (e.g., a header of the LCU 102) whereas the prediction mode type 110 is coded at the CU level (e.g., a header of the CU 104). However, since the delta quantization parameter 112 can only be signalled at the LCU level, this means that all the coding units 104 within the LCU 102 must share the same quantization parameter 112.
It is against this background that the present invention has been developed.